In lithographic printing systems, an original to be reproduced is scanned by a scanner on a pixel-by-pixel basis and the resulting scanned values are used to create one or more printing plates. When a monochrome (e.g., black and white) reproduction is to be printed, a single printing plate is produced. On the other hand, when a color reproduction is to be printed, a set of four plates are produced, one for each of the subtractive primary colors of magenta, cyan and yellow and one for black. The colored inks reproduce the hues of the original and the black ink produces a desired neutral density that cannot be attained by colored inks alone. In addition, since black ink is less expensive than colored inks, grey replacement (a form of undercolor removal) may be effected to replace quantities of the colored inks with black ink. Such a process reduces the cost to produce the reproduction without significantly affecting the appearance thereof.
In traditional prior art lithographic half-tone reproduction systems, each printing plate includes a number of contiguous cells of equal size wherein each cell contains zero, one or more elementary marks (or "microdots") clustered together to form a single large "dot" in the cell. More recently, systems have been devised wherein mircodots have been dispersed in a regular pattern in each cell and, in other systems, microdots have been dispersed in a random pattern on a medium. In each event, the number of elementary marks used to create a cluster dot or a dispersed dot depends upon the amount of ink to be applied to the substrate at the cell location. This is, in turn, dependent upon the scanned value of the original at a corresponding location thereof.
In the past, dots were formed within these cells on a regular spacing or grid using a screen in a photochemical etching process. More recently, half-tone reproduction systems have utilized data processing equipment that electronically produces data representing a half-tone image. This data can be used to plot film or to directly form a printing plate without the use of an actual screen. However, the terms "screen" and "screening" are still used to define the dot pattern produced in a half-tone reproduction. For example, the term "screen ruling" specifies the distance between centers of adjacent cells of the plate. When the cells are all of the same size and regularly spaced, the plate is said to have a "regular screening". In such a case, a cell contains one period of the "screen".
Systems that reproduce half-tone images with regular screening have several drawbacks. For example, resolution is limited by screen ruling. Screen ruling is limited, in turn, by the minimum dot size and spacing that can be reliably and consistently printed. Moreover, regular dot patterns produced by regular screening in color reproduction result in moire effects and color shifts caused by interference between the superimposed dot patterns. Such undesirable artifacts have been reduced in the past by superimposing the screens at angles with respect to one another. However, this technique is not entirely satisfactory since undesired effects are only minimized, not eliminated completely.
The prior art has reduced the effects of moire and color shifts while at the same time enhancing the quality of the reproduction by eliminating the use of regular screens. Instead, a process known as "screenless" lithography (also referred to as random screening or random dot lithography) has been used to produce irregular dot patterns on the printed page. The use of irregular dot patterns can eliminate visual interference caused by superimposition of the dot patterns, and hence moire effects are substantially reduced or eliminated.
In one prior art system, a printing plate having an irregular grain structure is photographically exposed and chemically developed in a photolithographic process to produce an irregular dot structure. Such systems cannot created consistent dot patterns from plate to plate, however.
Random screening has been electronically achieved utilizing an error diffusion technique, such as that described by Floyd and Steinberg in their paper "An Adaptive Algorithm for Spatial Greyscale", Proceeding of the S.I.D., Vol. 17/2, Second Quarter, 1976. This paper discloses a reproduction system that compares each continuous tone value, obtained by scanning an original, with a threshold to obtain a binary approximation of the continuous tone value. When the continuous tone value is less than the threshold, the continuous tone value is converted to a binary value of zero. If the continuous tone value is greater than the threshold, the continuous tone value is converted to a binary value of one. After conversion, the error resulting from approximation of the continuous tone value is subdivided into error portions and the error portions are summed in a prescribed pattern with neighboring continuous tone values yet to be converted so that such error is diffused. Each continuous tone value to be converted is thus a combination of its original continuous tone value plus any error portions diffused to it by the conversion of neighboring, previously converted continuous tone values. The next continuous tone value to be converted is then compared to the threshold and converted to one of the binary values. The resulting error, if any, is diffused to neighboring continuous tone values yet to be converted. This process is repeated until all continuous tone values resulting from scanning of the original have been converted to binary values. The binary values thus derived are used to produce a printing plate having dots at locations defined by such values.
While the foregoing process is effective to reproduce half-tone images with random dots, it has been found that the dots create artifacts in the reproduction. These artifacts detract from the visual appearance of the reproduction.
Sullivan, U.S. Pat. No. 5,051,844, discloses an error diffusion conversion system wherein a filter simulating the human visual system is utilized to reduce the incidence of artifacts in the reproduction. In this system, the filter comprises an array of filter elements or numbers, one of which is multiplied with an assumed binary value of zero for the continuous tone value undergoing conversion and the remaining of which are multiplied with binary values representing continuous tone values that have already been converted. The resulting values are subtracted from the continuous tone value undergoing conversion to produce a first error. A second error is produced in the same fashion, except that the one filter element is multiplied with an assumed binary value of one for the continuous tone value undergoing conversion. If the magnitude of the first error is less than the magnitude of the second error, the continuous tone value undergoing conversion is converted to a binary value of zero. Otherwise, the value is converted to a binary value of one. The error resulting from conversion of the continuous tone value is then diffused in a predetermined manner to neighboring continuous tone values that have not yet been converted.
The Sullivan system, therefore, simulates a retrospective spatial view of converted continuous tone values using a filter approximating the human eye in order to reduce perceived errors which would otherwise result.